Studies of Sea Surface Normalized Radar Cross Sections Observed by CloudSat

The primary science instrument of the CloudSat Mission is the Cloud Profiling Radar (CPR). The CPR is a W-band (94 GHz) nadir looking radar with the purpose of measuring backscattered power from hydrometeors (clouds and precipitation). Although the CPR contains an internal calibration system, external calibration using geophysical sources with known normalized radar cross-section (NRCS) values is also of interest. The ocean surface can potentially serve as one such geophysical source for external calibration if an accurate understanding of the sea surface NRCS versus the sea wind speed and direction and sea surface temperature is available.

Given that CloudSat operates in a near-nadiral observation geometry, it is typically assumed that a Geometrical Optics model for sea surface scattering should be applicable. In such a model, the sea surface is entirely described by its “long wave” slope variances. These slope variances are intended to describe properties of sea waves longer than a chosen “cutoff” wavelength, usually taken as some number of electromagnetic wavlengths (3 mm at W band.) Because this cutoff wavelength is a free parameter in the GO approach, it is important to develop alternative methods to avoid this ambiguity. A recently proposed “cutoff-invariant” two-scale model provides predictions that are invariant to choice of the cutoff wavenumber. This presentation will describe the implementation of such a model, and the simplification of this model into a GO prediction with an appropriate cutoff wavenumber when applicable.

Predictions of the model are compared with CloudSat observations primarily for nadiral measurements but also for periodic observations at incidence angles up to approximately 15 degrees. Required ancillary information for sea surface wind speed and sea surface temperature is obtained from the AMSR-E radiometer, and ancillary wind direction information from NCEP wind fields. Comparisons of models and CloudSat measurements are performed to assess the accuracy of existing sea spectrum models in predicting the long wave slope variances of the sea surface. Implications of the study for calibrating CloudSat measurements and for using CloudSat measurements to determine sea surface properties will be discussed.